Stretched surface recording disks (SD) generally consist of a rigid, circular support and a thin polymer film, having a recording layer, suitably attached to the periphery of the support. The development of SD has progressed over the years from a rather simple configuration described in U.S. Pat. No. 3,373,413 (Treseder) in which a film was stretched and clamped between two circular rings, to a dish-shaped support to which a stretched film was attached at the periphery as illustrated by U.S. Pat. No. 3,509,274 (Kilhara). Subsequently, U.S. Pat. No. 3,537,083 (Voth) introduced the concept of bonding the film at the center of the support in addition to the support periphery, and U.S. Pat. No. 3,599,226 (Lips) described an SD which included two stretched film surfaces, one on either side of the support, which were attached at the periphery and near the center of the support.
SD are superior to floppy disks in that a pressure pad is not required, and the recording surface has substantially the same overall dimensional stability as the relatively massive base to which it is attached. In addition, the SD provides a flat recording surface which may be deformed slightly to conform to a transducer head and irregularities in the surface of the head. Although the patents described above have illustrated the general configuration and certain desirable features of SD, practical problems remain as impediments to the mass production and general acceptance of SD. These include the problems of track dimensional stability and stress relaxation of the stretched film.
Track dimensional stability is an important parameter in the design of recording media and drive systems. A change in track shape can cause a serious loss in signal-to-noise ratio or the loss of prerecorded data. It has been determined that circular data tracks originally recorded on SD became elliptical or distorted as the SD were exposed to variations in environmental conditions. This track movement was not completely reversible, i.e., the original circular configuration was not obtained when the SD were returned to original environmental conditions. Thus track density is limited by the dimensional instability or anisotropy of the tracks.
It has been found that the tension or stress present in the stretched recording sheet decreases over time even without changing environmental conditions. This is referred to as stress relaxation and can also distort the data tracks.
U.S. Pat. No. 4,623,570 (Alexander, et al.) describes an SD which addresses these concerns by using a twice-annealed polymeric sheet as the substrate of the annular recording medium film. Data tracks recorded on SD made by this technique have significantly reduced track anisotropy when compared to SD made without twice-annealed polymeric sheets.
However, the present inventors have observed that if the radial tensions in the films of a two sided SD are not matched between the two faces of the disk at each point on the circumference of the SD, there will be an unbalance of radial tensions which will act to warp the SD. This unbalance is present, although to a lesser degree, even if the annealing techniques of Alexander, et al. are followed. In the worst case, where the diameters along which the disk support is warped toward the center of the disk are at right angles to each other, a saddle shaped disk is produced. The curvatures of the faces of the saddle shape vary with changes in the environmental conditions to which the disk is exposed. They also change when the environmental conditions are not changed, due to stress relaxation of the stretched film. In each case, distortion of the data tracks occurs.